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The cold air vs warm air issue will be more of a factor in carbed engines, then in FI engines, and again more of an issue in outside temps below 50 degrees F. In carbed engines cold air may even present drivability issues in extremely cold climates.
On a carbed engine , hot air allows the engine to run a leaner air/fuel mix, as the hot air tends to allow more fuel vaporization, and less of a need for the combustion chamber to use the actual burn in converting raw fuel into vapor , or useable fuel. Deterents to this process are the fact that the air and fuel mixed past the carb rarely spend more then .6 seconds in the intake before entering the combustion chamber,(hardly enough time to cause an appreciablle change of state in the fuel).
Hot air on an FI engine could easily help if the fuel is also heated,particularly on a wet manifold system or TBI, and this is doable because of the increased fuel pressure before the line, that will inhibit vapor lock. When the hot fuel is introduced to the air stream, the hot air will further help to keep it in a close to vaporous state as long as the puter does nothing to dump extra fuel into the mix.
Systems using hot air tend to run in a smoother fashion again because the air/ fuel mix is normally more homogenous.
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